IP multimedia subsystem (IMS) is a standardized architecture for provisioning of mobile and fixed multimedia systems. IMS provides for the transformation of the core wireless network architecture for efficient and cost-effective delivery of advanced multimedia services. Though driven by the 3rd Generation Partnership Project (3GPP), IMS is designed to be access agnostic—i.e., it enables access to advanced services via GSM, W-CDMA, CDMA2000, WiFi and other wireless broadband technologies like WiMAX, as well as fixed broadband systems. IMS aims to create a converged Internet Protocol (IP) infrastructure that allows key network resources to be shared by a wide range of services. IMS uses a voice-over-Internet Protocol (VoIP) scheme based on the standardized implementation of the session initiation protocol (SIP). The IMS standards define a network domain dedicated to the control and integration of multimedia services.
Without IMS, operators are forced to allocate dedicated components to each and every service that is deployed. Such service implementations are not scalable and are highly inefficient in a world in which users are demanding more personalized services and greater flexibility as regards to, for example, mixing and matching offerings. IMS also makes it possible for users to access multiple services in the course of the same call or session. Moreover, IMS enables operators to establish and control quality of service imperatives for individual services while at the same time protecting the overall integrity of network performance. IMS will significantly improve end-user experience by enabling flexible, personalized and seamless communications across access standards.
A multi-mode, multi-radio user equipment is capable of obtaining services through multiple radio networks, for example, via a wireless wide area network (WWAN) such as GSM/UMTS and via a wireless local area network (WLAN) such as 802.11 a/b/g. Of primary concern in the operations of such a multi-mode, multi-radio user equipment in low-power operations. With multiple radios in the user equipment, high power consumption will occur unless the operation of the radios and the protocol stacks are managed with low power operations in mind.
One of the main utilities of a multi-mode, multi-radio user equipment is its ability to obtain services from the network that best fits its needs. For example, if the coverage of GSM/UMTS is poor in an office building, but the coverage of WLAN is good, the user equipment may choose WLAN to make a voice call. As the user equipment moves from inside the office building to the outside, the coverage situation reverses, and unless the voice call may be transferred from the WLAN network to the GSM/UMTS network, the call may be disadvantageously discontinued.